The present invention relates to an objective lens for an optical pick-up of an optical disc drive, which is capable of reading/writing data from/to a plurality of kinds of optical discs whose cover layers have different thicknesses. In particular, the present invention relates to an objective lens formed with a diffractive lens structure on a surface of a refractive lens.
Generally, an optical pick-up for an optical disc drive is provided with a laser source, such as a laser diode, an objective lens which converges the laser beam emitted by the laser source on an information layer of the optical disc to form a beam spot thereon, and a signal detecting system that receives the laser beam which is reflected by the information layer of the optical disc and passes through the objective lens. The signal detecting system produces various signals based on the received laser beam.
There are various types of optical discs on which digital information is recorded at various densities, respectively. An effective diameter of an objective lens of an optical pick-up depends on an NA (numerical aperture), which is determined based on the density of the information recorded on the optical disc.
Generally, an objective lens used for a compact optical pick-up is made of a resin and is formed by means of an injection molding. A flow of material of a lens in the process of the injection molding is easily disturbed at an outer portion thereof and/or at a portion where the shape of the lens changes by a large amount. Therefore, in order to fabricate a lens having a necessary performance even at the outermost area within the effective diameter thereof, it is preferable that the lens has an outer diameter which is sufficiently larger than the effective diameter thereof, so that the change of the shape of the lens at the peripheral portion within the effective diameter area is prevented.
However, if the objective lens is optimized so that the above condition is satisfied, the optical performance of the lens at portions in the vicinity of the effective diameter may be continuous since the change of the shape is relatively small at the portions. This means that the effective diameter is substantially enlarged, which causes a bad influence on the formation of the beam spot on the optical disc. That is, if the effective diameter becomes greater, the NA becomes greater, which results in reduction of the size of the beam spot. Further, in such a configuration, if the optical disc inclines with respect to the objective lens, too much aberration occurs, which prevents recording/reading of the digital information on the optical disc.
However, if the peripheral portion outside the effective area of the objective lens is not optimized, due to the aberration caused by the peripheral portion, the size of the beam spot may become larger, which also prevents recording/reading of the digital information on the optical disc.
In order to avoid the above problems, in the prior art, an aperture stop member is provided on the light source side of the objective lens so that the beam incident on the objective lens is confined to the effective area thereof.
However, if the aperture stop member is located on the light source side of the objective lens, a size of an optical system of the optical pick-up along an optical axis direction increases, which prevents the downsizing of the optical system of the optical pick-up.
It is therefore an object of the present invention to provide an objective lens for an optical pick-up, with which light passing through the peripheral area, which is outside the effective area, does not have bad effect in forming a beam spot on an information layer of an optical disc without employing the aperture stop member.
For the object, according to the invention, there is provided an objective lens for an optical pick-up, which is configured such that at least one surface of the objective lens is an aspherical surface. The at least one surface is divided into an effective area and an outer area which is an area outside the effective area. The effective area and the outer area are formed such that a predetermined gap is caused between a spherical aberration of a light beam passed through the effective area and a spherical aberration of a light beam passed through the outer area. The light beam passed through the effective area forms a beam spot on a predetermined surface, while the light beam passed through the outer area is diffused on the predetermined surface.
With this configuration, due to the gap of the spherical aberrations of the beams respectively passed through the effective area and the outer area, the light passed through the outer area does not have bad effect in forming a beam spot on an information layer of an optical disc. Thus, if such a lens is used, it is unnecessary to employ the aperture member, and the optical performance thereof will not be deteriorated.
Optionally, a diffraction lens structure may be formed on the at least one surface within the effective area. The outer area may be connected with a base curve which is a macroscopic shape of the at least one surface within the effective area of the at least one surface.
Further optionally, the diffraction lens structure may include a plurality of concentric annular zones formed on said at least one surface. A step in a direction of the optical axis may be formed at every border between adjacent annular zones.
It is preferable that an absolute value of the gap of the spherical aberrations may be substantially equal to or greater than 10 micrometers.
In a particular case, the absolute value of the gap may be approximately 200 micrometers.
Still optionally, the surface of said outer area may be a continuous surface having no diffraction lens structure.
According to another aspect of the invention, there is provided an objective lens for an optical pick-up, at least one surface of said objective lens being an aspherical surface. The at least one surface may be divided into an effective area and an outer area outside said effective area. The effective area and the outer area may be formed such that the light beam passed through the effective area forming a beam spot on a predetermined surface, while the light beam passed through the outer area is diffused on the predetermined surface.